Re: function arguments optional default values

[Stephen Berman]

On Wed, 03 Jul 2024 21:21:34 +0000 Heime <heimeborgia@protonmail.com> wrote:

> Does elisp support function arguments optional default values ?

The answer is in the Elisp manual (info "(elisp) Argument List"):

     If actual arguments for the optional and rest variables are omitted,
  then they always default to ‘nil’.  There is no way for the function to
  distinguish between an explicit argument of ‘nil’ and an omitted
  argument.  However, the body of the function is free to consider ‘nil’
  an abbreviation for some other meaningful value.  This is what
  ‘substring’ does; ‘nil’ as the third argument to ‘substring’ means to
  use the length of the string supplied.
  
       Common Lisp note: Common Lisp allows the function to specify what
       default value to use when an optional argument is omitted; Emacs
       Lisp always uses ‘nil’.  Emacs Lisp does not support ‘supplied-p’
       variables that tell you whether an argument was explicitly passed.

Steve Berman